RCU: non-atomic assignment to long/pointer variables in gcc

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Documentation/atomic_ops.txt (182dd4b277177e8465ad11cd9f85f282946b5578)
says that pointers, longs, ints, and chars are stored and loaded atomically.

But GCC actually may split assignment to 'long' variable into two instructions.
see example in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981

GCC also splits assignments to 'volatile' variables and this is actually a bug in gcc.

volatile unsigned long y;

y = 0x100000001ul;

   400728:	c7 05 66 06 20 00 01 	movl   $0x1,0x200666(%rip)        # 600d98 <y>
   40072f:	00 00 00
   400732:	c7 05 60 06 20 00 01 	movl   $0x1,0x200660(%rip)        # 600d9c <y+0x4>
   400739:	00 00 00

fortunately for y = 0; it generates this:

   40071d:	48 c7 05 70 06 20 00 	movq   $0x0,0x200670(%rip)        # 600d98 <y>
   400724:	00 00 00 00

Thus NULL is safe, but constant ERR_PTR may be dangerous.

Probably rcu_assign_pointer() should use ACCESS_ONCE() around lvalue, because
splitting assignment for non-volatile variable seems like completely valid,
but this may help only after fixing that bug in GCC.